This invention relates to isocyanates and, more particularly, to a method for synthesizing isocyanates utilizing novel halosilyl carbamates and to the novel halosilyl carbamates.
While other techniques are known, the commercial manufacture of isocyanates is carried out almost exclusively by the reaction of amines with phosgene. The details of processing may vary somewhat with the specific isocyanate being formed, and in particular for aromatic and aliphatic isocyanates, but the general approach is the same. The use of phosgene is undesirable due to its toxicity as well as the care which must be utilized when employing it. In addition, the reaction conditions which are required restrict, to some extent, the type of isocyanates which may be prepared. The problems are multiplied in the manufacture of a diisocyanate, where the simple by-products may be intermolecular, e.g. -- a mixed carbamyl chloride/amine hydrochloride, and the urea by-products may be polymeric.
Various silicon derivatives have been employed previously to provide intermediates which break down to yield isocyanates. In Russian Chemical Reviews, Vol. 42, p. 669, (1973), a decomposition of an N-silylated carbamic ester under heating to provide isocyanates is disclosed: ##STR1## A dehydrative pyrolysis of a silyl carbamate is described in the translation of the Russian Journal of General Chemistry, Vol. 43, p. 2077 (1973), UDC 547,245: ##STR2## Still further, Angewandte Chemie, Vol. 70, p. 404 (1958), shows the formation of a highly substituted isocyanate from N-carbobenzoxy amino acid using trimethylsilyl chloride: ##STR3## In addition, in Angewandte Chemie, International Edition of 1968, at page 941, Vol. 7, there is described the thermal decomposition of nitrogen-substituted trimethylsilylated derivatives of carbamic acid esters, anhydrides, or chlorides. The reaction is shown below, Y referring to the acid derivatives previously mentioned: ##STR4##
None of the prior work shows the direct, continuous, synthesis of an isocyanate from an amine without the use of phosgene. Still further, in general, the reactions also require a relatively high forcing level of heating to yield the isocyanate.
It is accordingly an object of the present invention to provide a direct, continuous method for preparing isocyanates from amines without employing phosgene.
A further object provides a continuous method for the synthesis of isocyanates from amines which include other reactive, functional groups. A related and more specific object is to provide a method of preparing isocyanates from amines which also include hydroxyl, amino, mercapto, nitro, sulfonamido, amido or carboxylic groups.
Yet another object lies in the provision of preparing halosilyl carbamates.
Another object of this invention is to provide a method of preparing isocyanates in which the isocyanates are synthesized by heating intermediates under mild conditions.
A further object provides a method of preparing isocyanates which can be carried out in a single reaction vessel.
Other objects and advantages of the present invention will become apparent from the following detailed description.
While the invention is susceptible to various modifications and alternative forms, there will herein be described in detail the preferred embodiments. It is to be understood, however, that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modifications and alternative forms falling within the spirit and scope of the invention as expressed in the appended claims.